Optical coherence tomography (OCT) provides cross-sectional images of biological samples with resolution on the scale of several to tens of microns. Conventional OCT, referred to as time-domain OCT (“TD-OCT”), can use low-coherence interferometry techniques to achieve depth ranging. In contrast, Fourier-Domain OCT (“FD-OCT”) techniques can use spectral-radar techniques to achieve depth ranging. FD-OCT techniques have been shown to facilitate higher imaging speeds through improved signal-to-noise performance and elimination of a mechanically scanned interferometer reference arm.
FD-OCT systems generally operate by separating a light source into a sample beam and a reference beam. The sample beam can be directed at a sample to be imaged, and the reflected light from the sample is recombined with light from the reference beam (i.e., returning from the reference arm), resulting in an interference signal, which can provide information about the structure, composition and state, for example, of the sample. Light in the sample path and or light in the reference path can be modified by, for example, a phase modulator or frequency shifter, altering the characteristics of the interference and enhancing the information content of the signal or making the signal easier to detect. FD-OCT systems can sample the interference signal as a function of wavelength.
In one exemplary embodiment of the FD-OCT system, the interference signal as a function of wavelength can be obtained by using a light source that has an output wavelength which sweeps or steps as a function of time. A detection of the interference signal as a function of time thereby yields the interference signal as a function of wavelength. This exemplary embodiment can be referred to as optical frequency domain imaging (“OFDI”) technique.
In another exemplary embodiment of the FD-OCT system, the interference signal as a function of wavelength can be obtained by using a broadband light source and a spectrally dispersing unit or a spectrometer that spatially separates the recombined sample and reference light according to wavelength such that a one-dimensional or two-dimensional camera can sample the signal as a function of the wavelength. This exemplary embodiment can be referred to as spectral-domain OCT technique. In both such exemplary embodiments, the detected interference signal as a function of wavenumber k (k=1/wavelength) can be used to provide information related to the depth profile of scattering in a turbid or semi-turbid sample, or a transparent sample. Such information can include information regarding, e.g., the structure of the sample, composition, state, flow, and birefringence.
A scatterer at a given depth can induce a modulation in the amplitude or polarization of the interfered signal. The frequency of such modulation in wavenumber-space can be related to the location of the scatter or the time delay of the light reflected from that scatter relative to the time delay of the light in the reference arm. Scatterers located at a depth that causes reflected signals with no net time delay relative to the reference arm light can induce an interference signal that may not modulate with wavenumber. As the location of the scatterers moves from this zero-delay point, the magnitude of the frequency can increase. To image over large delay windows, e.g., to detect and localize reflections within large time delay window, the interference signal may often be sampled with sufficiently high resolution in wavenumber-space to facilitate an unambiguous detection of the range of modulation frequencies that are associated with the large delay window.
To accommodate the sampling at high resolution in wavenumber, increasingly fast analog-to-digital converters (“ADC”) can be used in the OFDI systems, and increasingly high pixel count cameras can be used in the SD-OCT systems. In both OFDI and SD-OCT systems, the increased data volume resulting from imaging over large extents can often result in the use of increasingly-high bandwidth data transfer buses and data storage units.
Thus, there may be a need to overcome at least some of the deficiencies associated with the conventional arrangements and methods described above.